Wave signaling system



(W. 26, 1935. w, A, MacDONALD 2,022,514

WAVE SIGNALING SYSTEM Filed May 25, 1928 2 She'ts-Sheet 1 fFFECT/VE/VfJS 550 K.C, I500 K-C FREQUENCY 55o KC 1500 Kc.

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WAVE SIGNALING SYSTEM Filed May 25, 1928 2 Sheets-Sheet 2 P v I QQN kg 7 RN 5 f R mm,

on H w a v I INVENTOR W/WmmA/Varflma/d BY W, ATIORNEY5 Patented Nov. 26, 1935 PATENT OFFICE WAVE SIGNALING SYSTEM William A. MacDonald, Little Neck, N. Y., as-

signor to Hazeltlne Corporation, Jersey City, N. 1., a corporation of Delaware Application May 25, 1928, Serial No. 280,464 26 Claims. (01.250-20) This invention relates to vacuum tube radio receivers and has to do more particularly with receivers intended forbroadcast reception and kindred purposes.

It has two primary objects, namely: (1) to provide an arrangement, devoid of complication, whereby radio waves of all frequencies within a predetermined band (such as the broadcast band) can be received with approximately uniform effectiveness; and (2) to provide an arrangement in which the several tunable circuits of a tuned multistage radio-frequency receiving amplifier may be operated through a common tuning control without their mutual resonance being adversely affected to any material extent by reason of the association therewith of the antenna circuit-and to do this without resorting to any expedient which, in itself, is objectionable. The successful accomplishment of these objects is a matter of cardinal importance in the evolution of broadcast receiver design because it provides at once the means of surmounting what, heretofore, have been considered two major obstacles lying in the way of the development of an entirely satisfactory receiving instrument embodying strictly unified tuning control--a feature which is now almost universally demanded in broadcast receivers.

To achieve these objects the antenna circuit is tuned to a fixed frequency of the order of the lowest frequency of the tuning range, whereby the antenna circuit is inductively reactive throughout the range. The antenna circuit is coupled directly to the first variably tuned circuit, either electromagnetically .or electrostatically, or both. If both kinds of coupling be employed they may either aid or oppose, depending upon whether it is desired to emphasize the higher or lower frequencies .of the range.

The present invention is concerned only with vacuum tube systems (more'particularly radio receivers including tuned radioefrequency amplifiers) having antennas of the untunable or invariably tuned variety. That is, the invention has no application to any system in which facilities are provided for directly tuning the antenna to each particular frequency which it is desired to receive. The invention is applicableespecially to systems (such as broadcast receiving systems) which are designed to operate over a comparativeiy wide band of frequencies, as for example, the present broadcast frequency range. The most important application of the invention is in broadcast receivers comprising two or more stages of tuned (that is variably tunable) radiofrequency vacuum tube relays or amplifiers in which all of the tuning condensers are mechanically interconnected and provided with a single tuning control device. On the other hand, the invention is not necessarily limited in its useful 5 application to the particularpurpose just stated.

It may, for example, be applied advantageously to radio receiverswhich do not have unified tuning control.

In so far as the broadcast receiver art is concemed there were, immediately preceding this invention, two widely recognized and accepted practices in vogue with respect to "the design of receiving antenna circuits and methods of coupling an antenna circuit to the input end of a tuned radio-frequency. amplifier arranged for unified control. One of these, which hasbeen perhaps the most generally employed, includes the use of a so-called coupling tube interposed between the antenna circuit and the first tunable circuit of the amplifier. The alternate prior art practice referred to consists in providing an antenna circuit which is resonant to a frequency of the order of the highest frequency to be received, and inductively coupling the first tunable 2 circuit thereto. The latter method or practice involves the use of an antenna coupling transformer having a primary winding of few turns and a large step-up ratio. Under the condition just defined, the antenna circuit has a capacitive reactance throughout the operating frequency range. As a result there is a strong inherent tendency for the antenna circuit to reflect capacity into the first tunable circuit. The reflected capacity is in effect added in parallel with the capacity of the first tuning condenser. Because of this phenomenon it is generally essential to the attainment of mutual resonance between the several tunable circuits, when they are operated through a common control device, to add padding condensers to each of the tunable circuits with the exception of the'first one. This, in turn, renders it necessary (in order to cover the whole wave-length range) to substantially inamplifier circuit due to variation in the reflected capacity. Hence a serious loss of sensitivity as well as selectivity is apt to result.

there is a cumulative effect resulting in a very pronounced accentuation of the higher frequencies at the expense of the lower frequencies. In. practice, where a capacitive type antenna circuit is employed, the effectiveness at the high frequency end of the operating range (where, for example, the range is of the order of the present broadcast band) is several times the effectiveness at the low frequency end. Inasmuch as uniform effectiveness throughout the operating frequency range is the ideal condition, it is apparent that the aforementioned widely variant effectiveness is objectionable.

The coupling tube method which has been mentioned may be said to give very satisfactory results under favorable conditions, but it is characterized by a serious and apparently unavoidable weakness whenever there are very strong interfering signal waves to be contended with. Since neither the antenna circuit nor the input circuit of a coupling tube is tuned to the frequency of the wave to be received, there is no initial discrimination against interfering waves of other frequencies; and as a consequence it often happens that a coupling tube will act as a detector, due to large grid-potential fluctuations brought about by interfering waves. That being the case, the wave which it is sought to receive is oftentimes modulated by the interfering wave or waves, and objectionable cross-talk results. The apparent effect is much the same as that due to poor selectivity. The condition referred to is most often experienced with receivers located near powerful transmitting actual observation it has been ascertained that the above described phenomenon occurs in a fairly large percentage of instances where coupling tubes are employed, and there is seemingly no way of avoiding it without eliminating the coupling tube.

The present invention is the outcome of extended study and investigation of the problems hereinbefore described. It avoids the use of a coupling tube with its concomitant cross-talk evil and consists, primarily, in the use'of an antenna circuit having an inductive instead of a capacitive reactance substantially throughout the range of predetermined operating frequencies. This results in two outstanding advantages, which are: a decidedly more uniform effectiveness with which waves of widely differing frequencies are received, and a radical diminution of the influence exercised by the antenna circuit upon the mutual resonance of the tunable circuits as compared with the influence in that respect of a capacitive antenna circuit.

An inductively reactive antenna circuit in ac-' cordance with the present invention does not reflect capacity into any of the tunable circuits of the amplifier. On the contrary, its effect is to subtract inductance from the first tunable circuit without disturbing its capacity. It would, perhaps, seem on cursory consideration that the ultimate product in either case would be the same; but there is a very simple and effective remedy available to correct the latter phenomenon, whereas there is not such an effective remedy available with respect to the former. All that is stations. From- .pling transformer. The alternate method in this necessary is to make the actual inductance of the first tunable circuit a little larger than the inductances of the succeeding tunable circuits. Then, when the subtraction of inductance by the antenna circuit occurs, the remaining effective 5 inductance of the first tunable circuit will be equal to that of the other tunable circuits and they will all be in resonance. The inductances not being variable tuning elements, as are the tuning condensers, there are no complications involved; and once the proper number of turns for the inductance of the first tunable circuit has been determined, the problem is definitely solved. The proper number of turns can easily be determined empirically. or calculated mathematically.

- This will be explained more fully in connection with the detailed description hereinafter.

With an inductively reactive antenna circuit, as prescribed in accordance with this invention, the dimensions of the antenna are far less criti- -cal than-in systems heretofore used. This is readily comprehensible when it is considered that the rcactance of the antenna circuit is mainly inductive, and that the inductance is a fixed quantity determined by the manufacture of the 2 receiver. For that reason the antenna dimensions and constants can vary considerably without causing a very large percentage variation of antenna circuit reactance and, consequently, without seriously influencing the first tunable 3 circuit. This does not mean that the length of the antenna is a matter of no consequence whatever. but it does mean that within certain rather wide limits the effect of changes in the aerial length is relatively very slight; and those limits 3 are such as to embrace any antenna which experience teaches is likely to be used for broad cast reception.

For the purpose of effectively dealing with the occasional instances where it may happen that the antenna-to-ground capacity is greater or less than the probable maximum or minimum, the invention contemplates and embraces means in the nature of a subordinate feature whereby supplemental capacity of suitable value may be in- 4 terposed in the antenna circuit in series with the inductance or in parallel therewith to offset, respectively, either of the two abnormal or unusual conditions referred to.

There are broadly two a ternative methods of applying to the antenna circuit the invention herein described. The first of these, and what is in general considered to be the preferred method, contemplates the employment of a coupling transformer whereof the primary winding which 5 is included in the antenna circuit has a relatively large number of turns. In this case substantially all the inductance of the antenna circuit is included in the primary winding of the cousents some practical advantages over the latter from the standpoint of manufacturing cost and from the further'standpoint of the utilization of inherent supplemental capacity coupling which may in many instances be accomplished most economically by the use of a large primary winding, as will be more precisely explained in connection with the detailed description to follow. 7

Likewise, with respect to the first tunable circuit to whichthe antenna circuit is coupled, there are two alternative and substantially equivalent methods of procedure .in applying the invention.

One of these contemplates providing on the coupling transformer a secondary winding having a larger actual sef-inductance than the desired effective inductance of the first tunable circuit. The alternative in this connection is to provide a secondary winding having a self-inductance which is equal or substantially equal to the desired effective inductance of the circuit and providing a supplemental or loading inductance in series therewith of -a value equal to the inductance which is subtracted as a result of the association of the antenna circuit with the first tunable circuit.

A detailed description of certain practicable applications of the invention together with a more extended and exact discussion of its operation and the improved resuts attainable will now be given with reference to the accompanying drawings in which,

Fig. 1 is a graph comprising two curves representing the voltage gain characteristics of two antenna circuits, one being capacitively reactive and the other inductively reactive; i. e., the comparative'effectiveness with which incoming signal waves of widely varying frequencies are received by invariably tuned antenna circuits having, on the one hand, (A) a high resonant period and consequently a capacitive reactance throughout the operating frequency range, and, on the other hand, (B) a low resonant period and inductive reactance;

Fig. 2 is a graph comprising two curves, one of which (C) illustrates the over-all transmission or amplification characteristic throughout a selected range of frequencies with a capacitively reactive invariably tuned antenna circuit, while the other curve (D) illustrates the over-all transmission or amplification characteristic throughout the same range of frequencies with an inductively reactive antenna circuit in accordance with the present invention;

Fig. 3 is a fragmentary circuit diagram of a vacuum tube radio receiver including a two-stage tuned vacuum tubeamplifier and detector to- .gether with an antenna circuit in accordance acteristic or degree of effectiveness with which wave energy throughout a wide range of frequencies, such as the present broadcast band, is impressed upon the input terminals of the first amplifier tube where the tunable input circuit of the latter is coupled directly to an invariably tuned antenna. The curve A represents in a general way the variant effectiveness with which wave energy of frequencies between 550 kilocycles and 1500 kilocycles (taken as the broadcast band) is impressed upon the input terminals of the first amplifier tube in an arrangement according to the prior art in which the antenna circuit comprises a primary winding having a small number of turns and is resonant at a frequency of the order of the highest frequency in the operating bandthat is to say, a capacitively reactive antenna circuit. Due to the fact that such an antenna circuit presents the lowest series impedance to the highest frequencies intended to be received, it follows as a matter of course that it discriminates vigorously in favor of the highfrequency end of the operating band. And since the succeeding amplifier stages inherently favor the higher frequencies, it is evident that here are two factors operating cumulatively to produce an over-all effectiveness of reception at the high-frequency end of the operating band which is greater than that at the low-frequency end. As a matter of fact, the effectiveness at the high-frequency end. under the condition stated, is ordinarily several times as great as the effectiveness at the lowfrequency end. Curve B of Fig. 1 denotes the salutary change of effectiveness with which the different frequencies are impressed upon the input terminals of the first amplifier tube as the result of using an antenna circuit of low series impedance at the low-frequency end of the operating range, in accordance with the present invention. Here it is seen that the effectiveness with which wave energy of 550 kilocycles is transmitted to the first amplifier tube is considerably greater than the effectiveness at 1500 kilocycles. It is immediately apparent from a comparative consideration of curves A and B that the over-all results between the two extremities of frequency are bound to be more nearly uniform when the antenna circuit is resonant. at or near the lower end of the frequency band.

The relative increments of signal strength at the two frequency extremes and intermediately, as they appear at the output end of a radio-frequency amplifier with which the present invention is employed are in a considerable measure subject to the control of the designer, but, nevertheless, dependent somewhat upon the number of stages of radio-frequency amplification uness some special means are utilized to counteract the characteristic tendency of radio-frequency amplifiers to accentuate the higher frequencies. For the instant purpose there will be considered only that type of radio-frequency interstage coupling which is in common use and which is inherently discriminatory in favor of the hi her frequencies.

On this basis the curves of Fig. 2 have been drawn to illustrate roughly the comparative overall results which may be considered to accrue, by combining, in the one instance, two factors having a tendency, individually, to accentuate the higher frequencies, and, in the other instance, two

factors having opposing tendencies in this respect. Curve C, Fig. 2 depicts a rapid rise in what is here termed over-all amplification, or transmission characteristic as the operating frequency increases. mission, as here employed. does not mean simply the measure of amplification between the input and output ends of a radio-frequency am-' The term over-all a'mplcation or transplifier, but involves in addition the influence of accordance with the practice heretofore followed in the design of radio receivers wherein no provision is made for tuning the antenna circuit to the wanted frequency and in which no coupling tube is employed.

Curve D, Fig.2, is intended to illustrate roughly an example of what can be accomplished in the way of modifying the over all amplification or transmission characteristic through the use of the present invention. In this case the over-all amplification or transmission is depicted as being greater at 550 kilocycles than at 1500 kilocycles. This curve has been purposely exaggerated somewhat in order to emphasize the sort of results that can be attained. But since uniformity of effectiveness throughout the operating frequency range is generally the desideratum, the curves of Fig. 2 are not exactly ideal; although curve D is, by comparison, a fairly close approximation to the ideal. It is evident, however, from these two curves and from the fact that the results are susceptible of considerable control by the designer, that a fairly close approach to perfect uniformity throughout the operating frequency range can be readily accomplished.

Fig. 3 is a circuit diagram illustrating a practicablc application of the invention to a vacuum tube radio receiver comprising atwo-stage transformer-coupled tuned radio-frequency vacuum tube amplifier and a vacuum tube detector. The audio-frequency or low-frequency portion of the receiver which would ordinarily be coupled to the output end of the detector has been omitted from the diagram. I

The radio-frequency amplifier tubes are identified by reference numerals I and 2, respectively, while the detector tube is identified by reference numeral 3. Each of the three tubes has a tunable input circuit designated integrally by reference numerals 4, 5 and 6, respectively. Each tunable circuit includes a variable tuning condenser and an inductancethe latter constituting the secondary winding of a radio-frequency coupling transformer. The inductances or secondary windings here referred to are designated by reference numerals I, 8 and 9, respectively, while the variable tuning condensers are designated by reference numerals I0, II and I2, respectively. The three tuning condensers should preferably be, and are assumed to be, identical, having the same minimum and maximum capacities and following the same law of capacity variation throughout their whole range of operation. It is to be assumed that the three tuning condensers are mechanically coupled together either by the use of a common shaft to which all the condenser rotors are connected, or by any other expedient means, several of which are well known in the art. The mechanical coupling. of the three condensers is indicated symbolically by the connecting lines designated as a whole by the reference numeral l3.

The output or plate-filament circuit of each of the amplifier tubes I and 2 includes the primary windings l4 and I5, respectively, of radiofrequency coupling transformers of which the coils 8 and 9 are the secondaries, respectively. It is plain that the interstage coupling is of the common radio-frequency transformer type and that it calls for little discussion beyond what is necessary to complete the description of the invention and explain what is believed to be the correct theory of operation. The amplifier illustrated is arranged to be neutralized in accordance with the well-known Hazeltine method of neutralization, and for this purpose neutralizing condensers I6 and I! are provided together with auxiliary coils i8 and I9, connected as described in United States Patent No. 1,533,858. The transformer is an audio-frequency transformer having its primary winding connected in the plate circuit of the detector tube. This transments will be explained later.

former couples theoutput side of the detector tube with the input side of the first audio-frequency amplifier tube. The latter is not included in the drawings.

In accordance with the explanation which has preceded there are two major objectives toward which the circuit illustrated in Fig. 3 is directed. One of these is to secure a uniform or approximately uniform degree of effectiveness with which all frequencies within the predetermined operating band or range are caused to appear at the output end of the radio-frequency amplifier, or in other words, at the input terminals of the detector tube. In this respect it may be assumed for the purpose of comparison that the incoming waves of different frequencies dealt with are of equal amplitude because it is not to be supposed that a wave of small initial amplitude will be received with the same effectiveness as one of large initial amplitude. In other words, it is to be kept in mind that when uniformity of effectiveness or uniformity of over-all amplification or transmission is referred to herein it is an implied condition precedent that the amplitudes of the received waves as they appear at the antenna, but not affected thereby, be equal.

The receiving antenna circuit 2! includes the primary winding 22 of a coupling transformer of which the inductance I of the first tunable circuit forms the secondary winding. In addition to the primary winding 22 the antenna circuit includes condensers 23 and 24 and an antenna switch 25. The purpose of the latter ele- There is also shown a condenser 26 having its terminals connected, respectively, to one terminal each of the primary and secondary windings of the antenna circuit coupling transformer. The antenna circuit, furthermore, includes a resistance element 21. This is a damping resistance which may or may not be utilized, as circumstances may dictate. Its function, when employed, is to cut down the resonance peak of the antenna circuit thereby contributing toward the accomplishment of uniformity of effectiveness and rendering the length of antenna employed even less critical. By thus decreasing the amplification and broadening the resonance peak, undesired signal voltages at the resonant frequency of the antenna circuit are diminished sufficiently to eliminate from the radio-frequency amplifier cross-talk and forced oscillations.

It is the intent, in accordance with this invention, that the antenna circuit should have such an inductance that under normal conditions it will be resonant at a fixed frequency of the order of the lowest operating frequency. For example, if the receiver is designed for broadcast reception the lowest operating frequency, according to the present prescribed broadcast band, is 550 kilocycles. The antenna circuit should, therefore, be resonant at or near 550 kilocycles. Under usual conditions it is preferable to make the antenna circuit resonant to a frequency slightly lower than the lowest operating frequency.

To obtain the necessary inductance to meet the prescribed condition the preferred practice is to give the primary winding 22 a relatively large number of turns, its self-inductance generally being considerably greater than that of the secondary winding 1; but substantially equivalent results may be obtained through the use of a supplemental inductance inserted in series in the antenna circuit together with a primary windreceiver.

ing having a relatively small number of turns. It is self-evident that if the antenna circuit is resonant at a frequency of the order of the lowest operating frequency it will discriminate in favor of the lower frequencies and against the higher frequencies, and that this discrimination will be the more marked the greater is the width of the operating frequency band. Consequently, wave energy of the lower frequencies in the broadcast band will be transmitted through the antenna circuit and thence to the input terminals of the first amplifier tube I with considerably greater effectiveness than wave energy of the highest operating frequency. Obviously, by a proper choice of antenna inductance in relation to the usual antenna capacity, the resonance frequency of the antenna circuit may be positioned in relation to the operating frequency band to vary within limits the effectiveness, or ratio of the voltage of the oscillatory circuit to the antenna voltage, in a preselected manner. The effectiveness, however, with which wave energy of the lower frequencies is successively amplified and transmitted through the interstage coupling systems is very much less than is the case with energy of the higher frequencies. That is, the antenna circuit and the interstage coupling systems have amplification characteristics which slope oppositely with frequency, the antenna characteristic being the inverse of, and complementary to, that of the interstage coupling systems. This point has already been fully expounded in connection with the graphs of Figs. 1 and 2 and would seem to require no further discussion.

Now will be considered the effects which the antenna circuit has or may haveupon the first tunable circuit 4 and indirectly upon the succeeding tunable circuits 5 and 6. Disregarding for themoment what has been said concerning the antenna circuit 2|, and assuming for the purpose of immediate discussion an antenna circuit having little inductance and hence a capacitive reactance throughout the operating frequency range, it may be correctly postulated that there will result a reflection of capacity from the antenna circuit into the first tunable circuit 4. The extent .to which this will occur depends, in part, upon the degree of coupling between the antenna circuit and the first tunable circuit and, in part, upon the step-up ratio between the primary and secondary windings of the antenna coupling transformer. The smaller the percentage of coupling and the higher the stepup ratio the less will be this reflection. In fact the reflected capacity is inversely proportional to the square of the turns ratio. That is one reason why primary windings of few turns have heretofore been utilized. But even with such a primary winding it is necessary to resort to very loose coupling if the reflected capacity is to be reduced to such an extent that it will have no material influence. Such extremely loose coupling introduces an excessive loss of sensitivity in the The alternative then, according to the better engineering practice, has been to compromise by permitting a certain amount of capacity to be reflected into the first tunable circuit, and compensating for it in the succeeding tunable circuits. The reflected capacity is simply added in parallel to the capacity of the tuning condenser l0, increasing the 'minimum capacity of the first tunable circuit. pensate for this condition it is necessary to add a like amount of capacity to each of the succeeding tunable circuits 5 and 6. The usual prac- To comtie has been to acid such capacity in the form of small padding condensers connected in parallel to the tuning condensers. Thus the minimumcapacities of the several tunable circuits are increased and equalized. This means, however, that the maximum capacitiesof the tuning condensers must be increased over and above what would otherwise be necessary by a comparatively large amount. In the case of a broadcast receiver designed to operate over a frequency band extending from 550 kilocycles to .1500 kilocycles the necessary increase of maximum capacity for each tuning condenser would be approximately nine times the supplemental capacity which is added, in the one case by reflection, and in the other by the supplemental or padding condensers. The maximum capacity of each tuning condenser must be increased by an'amount equal to approximately nine times the supplemental capacity because the ratio of maximum to mini- 20 mum capacity (where a fixed inductance is employed) must equal the square of the ratio of the vhighest; operating frequency to the lowest operating frequency. To illustrate, if the highest operating frequency in the broadcast band is 1500 kilo- 2. cycles while the lowest operating frequency is 550 kiiocycles, the ratio of high to low is roughly 3, and the square of this ratio is 9. This means, for example, that if the reflected capacity amounts to 10 micro-microfarads, the maximum capacity of each tuning condenser would have to be increased 90 micro-microfarads. Due to the need of compensation for reflected capacity, as hereinbefore described, it is not uncommonly necessary to increase the capacity of the tuning condensers as much as 25 or 30 percentover and above what would otherwise be required. This, of course, is a substantial factor in the cost of manufacture, and an important one in low-priced receiving apparatus.

Ii antenna circuits of small inductance, and hence of predominating capacitive reactance, couldbe relied upon from the standpoint of uniformity, the problem of attaining and maintaining mutual resonance between the tunable circuits under unified control would not be such a serious matter aside from the factor of cost. But such is not the case. It is evident that the reactance in a capacitive type antenna circuit, when dealing with frequencies of the present 50 broadcast band, consists for the major part in the inherent capacity between the antenna and ground; and since this is a factor subject to wide deviation as a result of the use of antennas of indiscriminate lengths, it is apparent that the 55 amount of capacity which will be reflected into the first tunable circuit is a very uncertain and widely variant factor. Hence, with such an arrangment (the present invention not being employed) the length of the antenna is a matter of 60 critical importance where it is sought to obtain the best results.

Let us now consider the performance of an inductively reactive antenna circuit such as that contemplated by this invention, and illustrated 65 in Fig. 3. In the first place, it has an effect upon the tunable circuit 4 which is distinctly different from that of a capacitively reactive antenna circuit. Instead of reflecting capacity into the encuit 4' it absorbs or subtracts inductance there- 70 from. This is equivalent to reducing the number of turns of the secondary winding 1. Its immediate influence is, of course, much the same as that resulting from the introduction of refiected capacity in that the mutual resonance between the tunable circuits is adversely affected. But because of the fact that the affected inductance is not the variable tuning element the adverse effect stated may be easily remedied by providing additional inductance in the first tunable circuit sufficient to offset the reduction or absorption of inductance therein which is produced by the antenna circuit.

Assuming that the coeflicient of coupling between the primary winding 22 and the secondary winding 1 is known or determinable, the amount of inductance which it would be necessary to add to the tunable circuit 4 in order to compensate for the aforesaid reduction is readily susceptible of calculation. The relation of actual to effective self-inductance in the first tunable circuit is expressible by the following equation:

In the foregoing equation 15 is the actual selfinductance of the tunable circuit 4 as measured with the primary winding 22 dissociated therefrom. LS2 is the actual as well as the effective self -inductance of the following tunable circuit 5, while L53 is both the actual and effective selfinductance of the next succeeding tunable circuit 6. It is essential that the effective self-inductance of the tunable circuit 4 be equal to that of the other tunable circuits 5 and 6. In the foregoing equation, (k) represents the coemcientof coupling between the antenna circuit and the secondary circuit. It is evident on its face that the denominator l-k is always less than unity, but

that if K is small, as it should be, the denominator is not much less than unity, and, accordingly, the effective self-inductance of the tunable circuit 4 is but little less than the actual self-inductance. In actual design it is usually necessary to make a compromise between optimum amplification and minimum detuning effect of the antenna circuit on the first tuned circuit coupled thereto. The preferred compromise coupling coefficient k is of the order of somewhat less than the square root of the natural power-factor of the secondary circuit. Such degree of coupling may be termed moderate. Knowing the desired effective self-inductance, the necessary actual selfinductance, as above defined, can be easily calculated from the foregoing equation. The secondary winding 1 should, of course, be designed to provide the calculated actual self-inductance when the antenna circuit exercises no influence thereon.

The required number of secondary turns can also be readily determined by trial; and, as a matter of fact, it will usually be necessary to make trial adjustments when the values have been determined by calculation.

Whether the antenna inductance consists entirely of the primary winding 22 or in part of a supplemental inductance in series therewith, it is evident that substantially all the inductance of the antenna circuit may form a part of the receiver as produced by the manufacturer, and may have a fixed value. It is further evident that the inductance constitutes by far the major portion of the total antenna reactance. Consequently, a variation of antenna length within any probable limits of variation will have a relatively small effect on the resonant period of the secondary circuii, and, as a consequence, the effect on the tunable circuit 4 will be very slight.

Investigation has shown that the capacities of receiving antennas provided by broadcast listeners range from an approximate minimum of 100 micro-microfarads to an approximate maximum of 300 micro-microfarads These are limits which are not likely to be exceeded in either direction, but may be in some cases. Within the limits of 5 variations stated, the total antenna reactance variation is not excessive where the antenna circuit is inductive, as herein described. If, however, an antenna of excessive length, that is, one having an excessive capacity, should for any reason be provided, its effect can be appropriately reduced through the use of a condenser 23 which through the operation of switch 25 can be connected in series with the primary winding 22. The capacity of the condenser 23 would then be in series with the capacity between antenna and ground and would serve to limit the over-all capacity of the antenna circuit. Thus, no matter how long the antenna might be the total series capacity of the antenna circuit would be always less than 2 that of the condenser 23.

On the other hand, an antenna might be provided which is entirely too short; and in anticipation of such a condition a parallel condenser 24 is provided. This condenser should prefer- 25 ably be of such capacity that the antenna circuit will have a resonant frequency somewhat below the lowest operating frequency irrespective of how short the antenna may be. Obviously the distributed capacity of coil 22 is to be calculated as a part of shunt capacity 24.

The switch 25 is shown in a position wherein the condenser 24 is connected in circuit inparallel with primary 22. There are three operating positions of the switch 25. The two additional positions are: First, that in which neither of the condensers 23 and 24 is operatively in circuit; and secondly, that in which the condenser 23 is connected in series with the antenna and the primary winding 22. In the latter case condenser 24 is not connected in circuit. With the arrangement described and illustrated in Fig. 3, no matter how long or how short the antenna may be its reactance will not vary sufficiently to cause any perceptible mistuning of the tunable circuit 4, providing, of course, that the switch 25 is in the proper position.

An arrangement in accordance with the disclosure of Fig. 3 is capable of being modified by the designer to provide a considerable choice of over-all results with respect to the relative effectiveness of widely different frequencies. Usually, it is preferred to secure, as far as possible, uniform effectiveness at all frequencies within the operating range, but that is not an unexcep- 5 tionable rule.

The condenser 26 connected as shown in Fig. 3 may be omitted, or may be utilized to supplement the inductive coupling between the antenna circuit and the first tunable circuit to effect a dual coupling therebetween. This condenser may serve to either aid or oppose the inductive coupling as may be desired. Whether it aids or opposes depends upon the relative direction of the turns of secondary winding I with respect to that of the primary winding; and its extent of influence is dependent upon its capacity. If the winding I is wound in one direction with respect to the primary winding the energy transmitted through condenser 26 will aid the inductively transmitted energy, whereas if wound in the other direction it will oppose. When the capacity coupling exactly opposes the magnetic coupling, the voltage applied upon the tunable circuit by the condenser is 180 out of phase with respect to that due to the magnetic coupling. Obviously, the effect of the condenser 26 increases with frequency. If, therefore, it is arranged to aid the inductive coupling it will tend to accentuate the higher frequencies more than the lower. But if it is arranged to oppose the inductive coupling it will tend to suppress the higher frequencies more than it does the lower. Therefore, it will be apparent that the response curve, such as curve D of Fig. 2, can be modified to a considerable extent, i. e., it can be made to slant in either direction or be substantially horizontal, by the provision of an ancillary coupling condenser such as condenser 26, in combination with an aiding or opposing secondary winding, such as inductance 1. Connection 35 extending between the low-potential terminal of primary winding 22 and that of secondary winding 7 provides a return path for current fiowing through-capacity 28. v

Since the amplification in a tuned multi-stage amplifier progresses geometrically, and since a transformer-coupled radio-frequency amplifier, such as that illustrated, discriminates in favor of the higher frequencies, it follows that the larger the number of radio-frequency stages the greater will be the disparity in degree of amplification between the two extremities of operating frequencies-especially where the operating frequency band is of considerable width. That being the case it may be very desirable whenv an extraordinary number of radio-frequency stages are employed to utilize a coupling condenser such as condenser 26 in opposition to the inductive coupling so as to discriminate against the higher frequencies. Whether or not the condenser 26 is employed is an arbitrary matter depending largely upon the exigencies of circumstances.

Fig. 4 illustrates an alternative arrangement with respect to that of Fig. 3. In this figure only the antenna circuit and the first amplifier tube are shown, since the remaining portion may be identical with the corresponding portion of Fig. 3. The antenna circuit in this instance includes two inductance elements 28 and 29 in seriesthe latter being the primary winding of the antenna coupling transformer. The combined inductance of coils 28 and 29 may be the same as the inductance of the primary winding 22 of Fig. 3. The primary 29 may consist of but a few turns while the coil 28 consists of relatively many turns. The antenna circuit is otherwise identical with that of Fig. 3.

The tunable input circuit 30 of Fig. 4 differs from that of Fig.3 by the inclusion of a supplemental inductance 31 in series with the secondary winding 32. The supplemental inductance 3| may be made equal to the inductance absorbed from the circuit 30 by the antenna circuit. The supplemental inductance 3i may be either fixed or adjustable in value. That is also true of the secondary windings in either instance.

There is sometimes an advantage to be gained through the use of the arrangement of Fig. 3 over that of Fig. 4 in that the adjunctive capacity coupling afforded by condenser 26 may be gained to a sufficient extent through utilization of the inherent capacity between windings. Since the primary winding of Fig. 3 is conceived to be larger than that of Fig. 4 it is possible, other things being equal, to secure a larger capacity coupling between primary and secondary through the inherent capacity alone than would be possible with a smaller primary winding. Thus with an arrangement otherwise in" accordance with Fig. 3. it may be practicable to omit the coupling condenser 26 and still secure the desired augmentation, whereas the same result might not be susceptible of accomplishment without the coupling condenser 26 in the case of an arrangement in accordancewith Fig. 4. 5

Inasmuch as the arrangement of Fig. 4 contemplates separating the antenna inductance into two distinct parts, and likewise the inductance of the first tunable circuit, it is evident that this is likely to lead to an increased manufacturing '10 cost. This is not thought, however, to be a very important matter except in the case of very lowpriced radio receivers wherein even the smallest savings are consequential.

The preceding description of the invention together with the rather extended discussion concerning its theory of operation should prove to be adequately clear and comprehensive to enable anyone skilled in the art to put the invention into practice. But in order to make the disclosure even more complete and specific some further data will be given with respect vto a particular example. For this purpose reference is now made to Fig. 3 wherein the following values may be employed: 2:;

Inductance of primary winding 22 .4190 milli-henries Actual self-inductance of secondary winding .2013 milli-henries Effective self-inductance of secondary winding 1....: .2000 milli-henries Co-eflicient of coupling between primary winding of 22 and secondary l Capacity of condenser 23-- Capacity of condenser 24. Capacity of condenser 2B .00001 to .00005 microfarads There is no need for prescribing values for any 40 of the other elements shown, since these are not in any way predicated upon the present invention and are readily determinable by anyone skilled in the art.

It goes without saying that this invention may be applied in a considerable variety. of forms in addition to those specifically described, and that its scope should be construed accordingly.

What is claimed is:

1. A coupling system, comprising inductance and capacity in an antenna circuit, for coupling said antenna circuit directly to a tunable input circuit of a wave signaling apparatus, said antenna-circuit capacity including the antenna capacity, the value of said inductance being such that said antenna circuit is resonant at a frequency fixed slightly less than the lowest frequency within the tuning range of said apparatus, whereby said coupling system discriminates in favor of the lower frequencies, means for connecting a capacity either in parallel or in series with the effective capacity of the antenna, whereby the effective antenna circuit capacity is maintained within limits such that the antenna circuit reactance is always inductive within said range.

2. The method of improving the degree of uniformity of effectiveness with which radio signal waves of widely different frequencies within a given frequency band may be received with a receiving system including an antenna system and a radio-frequency amplifier which amplifies the higher frequencies more effectually than the lower frequencies, said radio-frequency amplifier having at least one adjustably tunable circuit, which method includes the steps of magnetically 8 per cent .0003 microfarads .0001 microfarads coupling together the first adjustably tunable circuit of said amplifier and the antenna system, and fixedly tuning the antenna system to a frequency slightly lower than the lowest frequency of said band, thereby discriminating in favor of the lower frequencies sufiiciently to compensate for the tendency of said amplifier to discriminate in favor of the higher frequencies;

3. In a selective radio-frequency antenna coupling system an oscillatory circuit tunable over a desired frequency range, an antenna circuit including inductance intended to be connected to an antenna having a certain capacity, said antenna circuit including said antenna. capacity and having a natural frequency fixed slightly below said tunable range, coupling between said circuits, and resistance added in said antenna circuit whereby said system is made sufficiently unresponsive to interfering signals at the natural frequency of said antenna circuit.

4. In a tuned radio-frequency signaling system, a first coil and a second coil, like variable tuning condensers respectively connected across said coils, an antenna circuit having a natural frequency fixed slightly below the lowest frequency of the tuning range of said condensers and coils, and a moderate degree of coupling between said antenna circuit and said first coil whereby its effective inductance is reduced by a slight amount, the inductance of said first coil tance, said primary winding comprising a small being larger than the inductance of said second coil by the same said slight amount, whereby at any given signal frequency like tuning adjustments of said condensers are secured.

5. In a radio-frequency coupling network intended to couple a radio-frequency amplifier to an antenna having capacity, an oscillatory circuit tunable over a desired frequency range, and an antenna circuit coupled to said oscillatory circuit, the capacity and inductance of said antenna and said antenna circuit being resonant at a fixed frequency below said range, whereby the ratio of the voltage of said oscillatory circuit to the voltage of said antenna circuit varies in a preselected manner as the resonant frequency of said oscillatory circuit is varied.

6. In a radio-frequency coupling system intended to couple a radio-frequency amplifier to an antenna having capacity, a loading inductance, a transformer having a secondary tunable over a desired frequency range and a primary coupled to said secondary and adapted to be connected in the antenna circuit in series with said loading inductance, said primary winding comprising a small number of turns relative to the number of turns of said loading inductance, the capacity and inductance of said antenna and said primary and said loading inductance together being resonant at a frequency fixed below said range.

7. In a radio-frequency coupling system intended to couple a radio-frequency amplifier to an antenna having capacity, a transformer having a secondary tunable over a desired frequency range, a loading inductance, a primary electromagnetically coupled to said secondary and adapted to be connected in the antenna circuit in series with said loading inductance, said primary winding comprising a small number of turns relative to the number of turns of said loading inductance, an impedance in shunt with said series connected primary and loading inductance to decrease the detuning effect of the primary circuit on the secondary circuit, the capacity and inductance of said antenna and said number of turns relative to the number of turns of said loading inductance, the capacity and inductance of said antenna and said primary l5 and said loading inductance together being resonant at a frequency fixed below said range, and a resistance added in said antenna circuit whereby said system is rendered substantially unresponsive to interfering signals at the natural 20 frequency of said antenna circuit.

9. In a radio receiving system tunable throughout a frequency range and including a plurality of vacuum tubes in cascade relation, a first tunable coupling circuit for coupling an antenna to the first said tube, a second tunable coupling circuit linking a pair of said tubes, variable condensers for tuning said coupling circuits, and means in said coupling circuits causing the amplification of said first coupling circuit to vary 30 with the frequency of tuning in inverse and complementary order as compared with the corresponding amplification of said second coupling circuit, whereby in the operation of said system approximately uniform overall amplification is attained throughout said frequency range.

10. In a radio receiving system tunable throughout a frequency range, a plurality of cascaded vacuum tube amplifier stages of the type including means suppressing undesirable effects of capacitive coupling between control grid and plate of vacuum tube, each stage in cluding a tunable interstage coupling transformer, a tunable transformer for coupling the first of said vacuum tubes to a capacity-type antenna, each of saidtransformers having a primary winding magnetically coupled to a secondary winding, the effective values of inductance of all of the secondary circuits being of the same order of magnitude, the inductance of the primary circuit of at least one of said interstage transformers being less than the inductance of the secondary circuit coupled thereto, and the inductance of the primary circuit of said antenna transformer being greater than the inductance of the secondary circuit coupled thereto, whereby the overall amplification of said system is more nearly uniform than it would be if the values of inductance of the primary circuits of all of said transformers were of the same order of magnitude.

11. A coupling system for transferring electrical oscillatory energy, throughout a range of frequencies, between an antenna circuit and a variably tunable circuit inductively coupled 5 thereto, said antenna circuit including antenna capacity and fixed inductance of such value that the antenna circuit is inherently resonant at a fixed preselected frequency of the order of the lowest frequency of said range to which said 70 variably tunable-circuit may be made resonant, whereby the voltage gain characteristic of said coupling system discriminates in favor of the lower frequencies of said range.

12. A coupling system for transferring elec- 75 trical oscillatory energy, throughout a range of frequencies, between an antenna circuit and a variably tunable circuit dually coupled thereto, said antenna circuit including antenna capacity and fix-ed inductance of such value that the antenna circuit is inherently resonant at a fixed preselected frequency of the order of the lowest frequency of said range to which said variably.

. tunable circuit may be made resonant, whereby the voltage gain characteristic of said coupling system discriminates in favor of the lower frequencies of said range. v

13. In a radio-frequency coupling system intended to couple a radio-frequency amplifier to an antenna having capacity, a transformer having a secondary coil tunable over a desired frequency range, a loading coil, a-primary coil electromagnetically and electrostatically coupled to said secondary coil and adapted to be connected in the antenna circuit in series with said loading coil, said primary coil comprising a small numbar of turns relative to the number of turns of said loading coil, the capacity and inductance of said antenna and said primary and said loading coil together being resonant at a frequency fixed below said range, and a capacity in shunt with at least one of said coils in the antenna circuit whereby to decrease the detuning effect of the primary circuit on the secondary circuit.

14. In a wave signaling system, tunable over a certain frequency range, comprising a plurality of tunable oscillatory circuits coupled in cascade and including inductance and capacity of such values that the amplification characteristic curve of one of said circuits assumes a slope opposite to that of the amplification characteristic curve of the second of said circuits, whereby the output voltage of the second of said circuits is characterized by a substantially fiat voltage amplification characteristic curve.

15. In a wave signaling system tunable between the limits of a certain frequency range, two successive vacuum tubes each of which is preceded by a coupling circuit, the first of said coupling circuits including inductance and capacity of such values that said circuit is resonant at a frequency beyond one limit of said frequency range, the second of said coupling circuits including inductance and capacity of such values that said circuit is resonant at a. frequency beyond the other limit of said frequency range, whereby the voltage gain characteristic as observed at the output of said second vacuum tube is substantially uniform throughout said frequency range.

16. A radio receiver comprising thermionic tubes, tunable circuits each having a single adjustable tuning device, structure mechanically.

connecting said tuning devices for adjustment in unison to like extents, an antenna circuit coupled to one of said tunable circuits, and an inductance in said antenna circuit for tuning it to a wavelength higher than the highest wavelength of the tuning range of the receiver, said inductance having its resonant period outside of said range.

17. In a system for the reception of signals of carrier frequency, a source of signal energy, amplifying and translating means having non-uniform sensitivity for different carrier frequencies impressed upon the same, and means interposed between the source and said amplifying and translating means for repeating signal energy from c uniform sensitivity for different carrier frequencies of a. character tending to compensate for the non-uniform sensitivity of the amplifying and translating means.

18. In a system for the reception of signals of carrier frequency, a source of signal energy, a radio frequency amplifier, detecting and translating means receiving energy from the amplifier, said amplifier having non-uniform sensitivity for signal energy of different frequencies, and 10 means including a vacuum tube relay interposed between the source. and said amplifier for repeating signal energy from said source to said amplifier, said last named means having nonuniform sensitivity for signal energy of different frequencies but of a character tending to compensate for the non-uniform sensitivity of the amplifier.

19. In a system for the reception of signals of carrier frequency, a source of signal energy, a receiver including a plurality of vacuum tube relays coupled'in cascade and having a plurality of individual tuning elements simultaneously variable by a unitary control whereby the receiver may be selectively'tuned over a wide range of radio frequencies by movement of said unitary control, said receiver having a non-uniform transmission characteristic for energy of different frequencies, and means interposed between said receiver and said source for preventing changes in the constants of said individual tuning elements due to variations in the electrical properties of said source, said means serving to repeat signal energy from said source to the receiver in such a manner as to compensate for the non-uniform transmission characteristic of the receiver.

20. In a system for the reception of signals of carrier frequency, a source of signal energy, a receiver including a plurality of vacuum tube relays. coupled in cascade and having a plurality 40 of individual tuning elements simultaneously variable by a unitary control whereby the receiver may be selectively tuned over a wide range of radio frequencies by movement of said unitary control, said receiver having a non-uniform response characteristic for energies of different frequencies, and means interposed between said receiver and said source for rendering said individual tuning elements independent of variations in the electrical properties of said source, said means comprising a vacuum tube relay having its output circuit coupled to the input circuit of the receiver, and a plurality of reactive elements associated with the input circuit of said relay and the source of signal energy, said elements being so arranged as to automatically vary the effective electrical coupling between said source and said relay in accordance with a variation in the frequency of signal energy, said variable electrical coupling serving to compensate for the non-uniform: response characteristic of the receiver.

21. A radio frequency amplifier tunable over a range of frequencies and having non-uniform sensitivity over said range, a signal energy input device and means interposed between said amplifier and said input device to modify the efficiency of the latter compensatively with respect to the sensitivity of said amplifier for different frequencies.

22. A radio receiving system comprising a. plurality of resonant circuits, tuning devices therefor having adjustable elements for tuning said system throughout a frequency range, an an tenna circuit coupled to one of said tunable cir- '75 cults, and means to permit like extents of movement of the adjustable elements of said devices in tuning from resonance at one frequency to resonance at another frequency comprising a coil in said antenna circuit of a value insuring that said antenna circuit is inductively reactive throughout said range.

23. A radio receiving system comprising cascaded vacuum tube amplifiers, circuits coupling said amplifiers tunable throughout a frequency range, an antenna circuit, a tunable circuit interposed between said antenna circuit and the first of said amplifiers, tuning devices for said tunable circuits, and means to increase overall ampliflcation of said system and to permit adjustment of said tuning devices in unison to like extent comprising a coil in said antenna circuit of such value that said antenna circuit is inductively reactive throughout said range.

24. In a radio-frequency receiving system, am

input circuit tunable over a certain frequency range, an antenna circuit adapted to be connected to an antenna having capacity, and an inductance included in said antenna circuit of such value that said antenna circuit is resonant at a frequency of the order of the minimum frequency of said range, said antenna circuit and said input circuit being electromagnetically and capacitively coupled to control the transmission characteristics of said system.

25. In a radio-frequency receiving system, an input circuit tunable over a certain frequency range, an antenna circuit adapted to be connected to an antenna having capacity, and an inductance included in said antenna circuit of such value that said antenna circuit is resonant at a frequency of the order of the minimum frequency 1 of said range, said antenna circuit and said input circuit being electromagnetically and capacitively coupled in aiding phase to control the transmission characteristics of said system.

26. In a radio-frequency receiving system, an 15 input circuit tunable over a certain frequency range, an antenna circuit adapted to be connected to an antenna having capacity, and an inductance included in said antenna circuit of such value that said antenna circuit is resonant at a go frequency of the order of the minimum frequency of said range, said antenna circuit and said input circuit being electromagnetically and capacitively coupled in opposing phase to control the transmission characteristics of said system. 25

WILLIAM A. MACDONALD. 

